In­Sight – the end of a sci­en­tif­ic suc­cess sto­ry

The InSight Mars mission is history. On 20 December 2022, NASA declared the mission over. The two attempts from Mission Control Centre at NASA's Jet Propulsion Laboratory (JPL) in Southern California to reach the lander via relay satellites in Mars orbit have been unsuccessful. This almost certainly means that InSight's solar-powered batteries are no longer supplying enough power, a condition engineers call 'dead bus' mode. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) contributed measuring instruments and a science team to the lander mission. InSight was the first purely geophysical mission to explore Mars. The last radio contact with Earth took place on 15 December 2022.

InSight uses solar energy to recharge the batteries, which is currently no longer sufficiently possible due to the dust accumulation on the solar panels. If, however, wind cleared the panels and a sufficient charge level is reached again, InSight could power up and attempt to communicate. Further contact would then be possible and even a resumption of operation. However, the increasing dust build-up on the solar panels makes this unlikely.

"It is always regrettable when a planetary mission for which you have prepared for more than a decade and then operated for years finally fails to deliver further measurement data," says Heike Rauer, Director of the DLR Institute of Planetary Research in Berlin, looking back on the InSight mission. "On the other hand, the positives absolutely outweigh the negatives: the scientific fruits of preparation and planning have been successfully harvested. We have learned a lot about the internal structure of Mars and are also using this to understand the other Earth-like bodies in the Solar System. Our planetary geophysicists have drawn many important lessons from the measurements."

The end of the mission loomed over the course of the last few months and came as no surprise. After more than four years, the mission duration had exceeded expectations by a factor of two. The Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport (InSight) mission was NASA's eighth mission to land on Mars since 1976 and the first to be devoted almost exclusively to geophysical investigations. The solar panels were designed in such a way that they would provide enough energy for the originally planned lifetime of one Mars year (two Earth years), despite dust accumulation. In the end, they lasted long enough to extend the mission duration by a second Mars year.

Almost everything succeeds as planned

The focus of the mission was to measure the heat balance and seismic activity inside the planet in order to gain important information about its structure, the flow of heat from the core and mantle to the surface and, derived from this, its thermal evolution. The primary instruments for acquiring these measurements were the Heat Flow and Physical Properties Package (HP³) provided by DLR and the Seismic Experiment for Interior Structures (SEIS) seismometer developed by the French space agency CNES. It is the NASA Mars mission with by far the most significant European contribution to date. NASA Science Director Thomas Zurbuchen hailed the mission as a great success.

Robotic arm helps mole dig below the surface

Unfortunately, the affectionately named Mars 'Mole' probe of DLR's HP³ did not fully deliver the expected measurements (click here to read the logbook of Principal Investigator, Tilman Spohn, former Director of DLR's Institute of Planetary Research), because the heat flow probe could not successfully penetrate to the required depths beneath the Martian surface. HP³ and the Mole kept the team busy for more than two years. Originally, the Mole was supposed to penetrate to a depth of five metres and drag a tether with temperature sensors behind it. "This would have allowed us to measure how the temperature rises with depth. With the help of the thermal conductivity measured during the Mole's descent, we could have directly determined the heat flow from the interior of Mars," explains Tilman Spohn. "This would have helped us to classify the evolution of Mars from a hot origin to its current, almost entirely cold state."
The Mars Mole, designed as a self-hammering probe capable of penetrating through the familiar loose, sandy soil of other missions, was unable to find a foothold in the unexpectedly hard soil around InSight's surroundings. The instrument was eventually able to bury its 40-centimetre probe just below the surface, collecting some valuable data on the mechanical and thermal properties of the martian soil despite its setback. "These data will be very helpful for future exploration of Mars by humans or robots trying to dig into the Martian subsurface," Tilman Spohn continues. The fact that the Mole was finally able to burrow in is thanks to a team effort by engineers from JPL and DLR. They used the InSight lander's robotic arm in a creative way to give the Mole additional support. The arm and its small scoop were primarily designed to place scientific instruments on the surface of Mars. Eventually, though, they even helped clear some of the dust from InSight's solar panels as the power waned.

Seismometer provides 'ground-breaking' data

In the SEIS experiment, however, the successful recordings of waves propagating through the Martian crust were of enormous scientific value. Between early 2019 and the end of the mission, seismic waves from more than 1300 'events' - tremors in the martian soil - were recorded. These include mostly waves generated by marsquakes that occurred at different locations in the Martian crust during the discharge of tectonic stresses, but some of these waves were triggered by asteroid impacts. In these cases, even the location of the impacts could be reconstructed from the data and, in several cases, confirmed using images acquired by the Mars Reconnaissance Orbiter. The two largest related craters measure more than 100 metres in diameter.

Caused by tectonic activity, these marsquakes provide important clues about the structure of the Red Planet. The reflection of these seismic waves at the boundaries between the solid rock mantle and the liquid core finally allowed the size of the martian core to be determined precisely. Its diameter is between 3600 and 3700 kilometres, which is at the upper end of the size estimated before the mission. For comparison, the total diameter of Mars is just under 6800 kilometres. Seismic waves passing through the core also provide clues to its internal structure and composition. The complementary auxiliary instruments on board also provided important data, such as the DLR RAD radiometer, which is part of HP³. RAD recorded the daily change in the surface temperature by measuring infrared radiation. This made it possible to collect important data for characterising the thermal properties of the martian soil.

Patience required - quakes limited to the Martian summer

When the seismometer went into operation at the beginning of 2019, not a single initial Mars quake was detected in the recordings for several weeks - much to the alarm of the InSight team. "We were already calculating what it would mean for our theories not to register any quakes," recalls Martin Knapmeyer, a seismologist at DLR's Institute of Planetary Research who is involved in the SEIS experiment, conveying the nervousness felt by the team in the first weeks of the mission in 2019. "When things did get going much later, it became clear that the noise of the wind in the region of the InSight landing area, where it was currently winter, masked all signals of marsquakes. On top of this, we were later able to prove that the frequency of these quakes is actually lower in winter than in summer."

Later, on 'warm' spring and summer evenings in the Elysium Planum region in which InSight landed, there was almost no wind. This made for ideal measurement conditions, primarily between sunset and midnight. Ultimately, more than 1300 marsquakes could be registered. Many of them originated in the Cerberus Fossae region, 1500 kilometres away from InSight. This corresponds roughly to the distance between Cologne and Mount Etna in Sicily. In the Cerberus Fossae region, the last volcanic activity took place less than 200,000 years ago, and the observed quakes exhibit characteristics similar to those of volcanic regions of the Earth, such as the Eifel region in Germany. "However, this does not mean that a new volcanic eruption is to be expected here in the near future," Knapmeyer adds, assessing the measurements.

At last, numerical values for the thickness of the Martian crust

From the study of variations in the Red Planet's gravitational field, which alter the altitudes of Mars orbiters by tiny amounts, it has long been known that the martian crust has a regionally varying thickness. For decades, efforts have been made to use seismic measurements to measure not only the relative but also the absolute thickness of the crust. With InSight, this has now been achieved. Initially this was only done for the landing site. However, by registering surface waves from some of the stronger marsquakes, it also became possible to determine the crustal thickness along the path of these waves. "This means that numbers can now finally be attached to the 'contour lines' of the crust thickness," says Knapmeyer, highlighting another important result of the mission. The average thickness of the crust is between 24 and 72 kilometres, which is somewhat thinner than earlier, more indirect investigations had shown.

Diverse missions on Mars

There are three active on the surface of Mars: NASA's Curiosity rover, which landed in 2012 in the Gale crater; the Perseverance rover, which landed in Jezero crater in 2021, and the Chinese mission Tianwen 1, which comprises the Zurong rover and landing station. In orbit around Mars are NASA's Mars 2001 Odyssey probe (since 2001), the European Space Agency (ESA) Mars Express orbiter (since 2003), equipped with DLR's High Resolution Stereo Camera (HRSC), NASA's Mars Reconnaissance Orbiter (since 2006), NASA's MAVEN atmospheric orbiter (since 2014), ESA's ExoMars Trace Gas Orbiter (since 2016), the orbiter of China's Tianwen-1 mission (since 2021), and the United Arab Emirates' Al-Amal orbiter (since 2021).